This invention relates generally to vortex-shedding flowmeters adapted to measure flow rate, and more particularly to a meter capable of accurately measuring the flow rate of gases as well as liquids, the meter including a dual-body vortex shedder.
It is well known that under certain circumstances the presence of an obstacle or shedder in a flow pipe will give rise to periodic vortices. For small Reynolds numbers, the downstream wake is laminar in nature, but at increasing Reynolds numbers, regular vortex patterns are formed which are known as Karman vortex streets. The frequency at which these vortices are shed is a function of flow rate.
This phenomenon is exploited to create a flowmeter for measuring the volumetric flow of fluids being treated or supplied in order to carry out various control functions. Flowmeters operating on this principle are disclosed in the Bird patent 3,116,639, and in the White patent 3,650,152. Flowmeters of the vortex-shedding type, such as those disclosed in the Burgess patent 3,888,120 and the Herzl patent 4,162,238, are capable of effecting accurate volumetric or mass flow measurement.
The Herzl patent 4,181,020 discloses a vortex-type flowmeter in which fluidic oscillations produced by a shedder mounted in a flow pipe are sensed by a downstream balanced-vane sensor pivoted in a torsional suspension that allows only microscopic vane motion. The shedder acts to divide the incoming fluid flowing therethrough and causes vortices to be shed alternately on either side thereof. The downstream train of vortices passing on either side of the vane sensor generates fluidic forces giving rise to alternate clockwise and counterclockwise torques, causing the sensor to oscillate mechanically at a frequency proportional to the flow rate of the fluid being metered.
The Herzl patent 4,226,117 discloses a vortex-shedding flowmeter wherein torsionally-supported behind the shedder is a drag-actuated sensor which includes a pair of parallel legs symmetrically disposed with respect to the longitudinal axis of the flow pipe.
With a drag-actuated sensor, as vortices are successively detached from the shedder and appear alternately on either side of the gap between the shedder and the downstream sensor, the low pressure region generated by each vortex acts to displace the stagnant zone produced in this gap as a result of fluid flow past the shedder to a position in front of the adjacent leg of the sensor, the fluid flow then going around and past the other leg, thereby developing a torque about the pivot axis. These torques are developed alternately, causing the torsionally-supported sensor to oscillate at a frequency in accordance with flow rate.
In both the Herzl patents 4,181,020 and 4,226,117, motion of the torsionally-supported sensor is detected by means of a transducer which takes the form of a strain gauge bonded to a resilient beam, one end of which is attached to the trunnion or shaft of the sensor projecting through the flow pipe, the other end being anchored. The resultant deformation of the beam as the shaft oscillates is translated by the strain gauge into a corresponding electrical signal whose frequency is indicative of flow rate. As pointed out in these Herzl patents, an important advantage of a vortex flowmeter having a torsionally-mounted sensor is that the meter is effective and accurate for both liquid and gas flow measurements.
The drag-actuated sensor arrangement disclosed in Herzl patent 4,226,117 closely approaches the ideal requirements for a sensing system in a vortex-type flowmeter. These ideals are as follows:
A. The system has a sensitivity which renders the meter effective for low-pressure gas measurement.
B. The system is one which has an inherent ruggedness that renders the meter suitable for heavy-duty liquid flow rate measurement.
C. The system is insensitive to mechanical shock and acceleration forces to which the flowmeter is subjected.
D. The system is capable of operating over the broad temperature range normally encountered in gas and liquid measurement and is capable of operating over a very wide operating frequency range.
E. The sensing system requires virtually no motion and is not limited by torque transducer bonding or attachment problems.
F. Finally, the sensing system is one which is relatively inexpensive and has a compact structure.
Flowmeters of the type disclosed in Herzl patent 4,266,117 operate reliably and efficiently in relatively large meter sizes for measuring the flow rate of gases or fluids. But in smaller sizes, as with meters having an internal diameter of two or three inches, the fluidic forces which act on the torsionally mounted, drag-actuated sensor are relatively weak, with a resultant low output signal that is difficult to distinguish from noise signals.
Moreover, with a small vortex meter of the drag-actuated type whose operation depends on a slot in the rear body, this slot will necessarily be relatively narrow in a small meter and therefore subject to blockage by contaminants carried by the fluid being metered.